The present invention relates to a magnetoresistive device and a method for producing the same, and a magnetic component.
A TMR (tunnel magnetic resistance) device is a device in which a very thin insulating layer is inserted between two ferromagnetic layers. A TMR device uses the phenomenon that the tunneling current flowing through the insulating layer is changed by the relative angle of magnetization of each metal element M.
It has been expected in theory that when using a ferromagnetic metal element M having a high spin polarizability such as Fe or FeCO for the ferromagnetic layers, a high rate of change in magnetic resistance of at least 35% is obtained (M. Jullier, Phs. Lett. 54A (1975) 225). However, a high MR (magneto resistance) has not been possible to realize.
Recently, Miyazaki et al. reported that they produced an insulating layer of alumina by natural oxidation in the air, and obtained a high rate of change in MR (T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231). With this report, active development of TMR materials and TMR devices has started.
The recently reported methods for producing insulating layers showing a high MR are classified largely into two methods. One is a natural oxidation method in which an aluminum film formed on a ferromagnetic film is oxidized in the air or in pure oxygen (Tsuge et al., Document of the 103rd Workshop by the Society of Applied Magnetics of Japan, p. 119, (1998)). The other is a plasma oxidation method in which an aluminum film formed on a ferromagnetic film is oxidized in an oxygen plasma (J. S. Moodera et al. Phy. Rev. Lett., 74, 3273 (1995)).
To obtain a high MR, these TMR devices use a transition metal showing a high spin polarizability such as Fe or CoFe for the lower ferromagnetic layer on which the aluminum film is formed.
Because the current flowing in a TMR device is mainly a tunneling current through an insulating layer, the resistance of the device is substantially high. Thus, when a TMR device is used as a reproducing head or MRAM, S/N ratio decreases due to thermal noise, and threshold frequency of a readout circuit decreases during a fast response.
To lower the resistance of the device, reducing the film thickness of the alumina insulating layer could be considered. However, with a conventional process for oxidizing an aluminum film, the lower ferromagnetic film is likely to be oxidized beyond the aluminum film when the aluminum film is thin. As a result, when antiferromagnetic materials such as Fe2O3 and CoO are formed at the interface with the aluminum oxide film by an excess oxidation reaction, for example, due to the interaction with these antiferromagnetic oxides, tunneling electrons lose information of magnetization direction with an external magnetic field.
On the other hand, when the aluminum film is not oxidized completely and a portion of the aluminum film remains, the spin memory of the tunneling electrons passing through the remaining aluminum film is lost, and MR is reduced.
Furthermore, in conventional TMR devices, when a large bias is applied, the rate of change in MR is decreased greatly due to generation of magnon, etc.
Furthermore, conventional MR devices do not have a sufficient thermal stability, and for example, when using them as MRAM, heat deterioration such as decreased MR property is caused during post-annealing of CMOS (at about 250 to 400xc2x0 C.) or heating in the production of MR heads (at about 250xc2x0 C.), or during its use.
In view of the above-mentioned problems, it is an object of the present invention to provide a new magnetoresistive device having a low junction resistance and a high MR and a method for producing the same, and a magnetic component.
To achieve the above object, the present invention provides a magnetoresistive device including a high-resistivity layer, a first magnetic layer and a second magnetic layer, the first magnetic layer and the second magnetic layer being arranged so as to sandwich the high-resistivity layer, wherein the high-resistivity layer is a barrier for passing tunneling electrons between the first magnetic layer and the second magnetic layer, and contains at least one element LONC selected from oxygen, nitrogen and carbon; at least one layer A selected from the first magnetic layer and the second magnetic layer contains at least one metal element M selected from Fe, Ni and Co, and an element RCP different from the metal element M; and the element RCP combines with the element LONC more easily in terms of energy than the metal element M. In the magnetoresistive device of the present invention, the element RCP contained in the layer A combines selectively with the element LONC diffusing from the high-resistivity layer to form a compound. Thus, in the magnetoresistive device of the present invention, oxidation, nitriding or carbonization of the metal element M can be inhibited, thereby preventing generation of a localized spin resulting in spin inversion. Furthermore, when the element RCP in the layer A combines with the element LONC to form a compound, the compound itself functions as a part of the high-resistivity layer. In addition, because the diffusion velocity of oxygen ions or nitrogen ions in the compound of the element RCP and the element LONC is substantially lower than in the magnetic films, the compound of the element RCP and the element LONC acts as a layer to inhibit diffusion of excess oxygen or nitrogen. Therefore, in the magnetoresistive device of the present invention, formation of a high-resistivity layer having a larger thickness than necessary, resulting in an increase in the resistance of the device, is inhibited. Thus, according to the magnetoresistive device of the present invention, a device having a low junction resistance and a high MR is obtained.
In the magnetoresistive device of the present invention, it is preferable that the layer A contains the element RCP so that a concentration of the element RCP is high on the side of the high-resistivity layer. When the element RCP is in a solid solution state with a metal composed of the metal element M, its spin polarizability is generally lower than that of the single metal composed of the metal element M. However, by making the concentration of the element RCP high on the side of the high-resistivity layer in the layer A, the element RCP forms a compound, and the element RCP and the metal element M are separated in phase. As a result, high spin polarizability is obtained in the vicinity of the high-resistivity layer, which has the greatest influence on the rate of change in magnetoresistance. The spin polarizability can be increased by decreasing the concentration of the element RCP as it is farther from the high-resistivity layer. The layer A may have a two-layered structure such as an element RCPxe2x80x94containing Fe layer/Fe layer from the side of the high-resistivity layer. The layer A also may have a structure such as an element RCPxe2x80x94containing FeCo layer/FeCo layer from the side of the high-resistivity layer.
In the magnetoresistive device of the present invention, it is preferable that the element RCP is at least one element selected from Si, Ge, Al, Ga, Cr, V, Nb, Ta, Ti, Zr, Hf, Mg and Ca. These elements have a larger free energy in negative for forming oxides than the metal element M, and selectively capture oxygen ions or nitrogen ions diffusing from the high-resistivity layer. Among these elements, Si, Al, Cr and Ti have particularly large diffusion constants in metal elements. Thus, these four elements diffuse toward the side of the high-resistivity layer so that their concentrations are high on the side of the high-resistivity layer, using as a driving force the chemical potential gradient of the oxygen or nitrogen ions generated when forming the high-resistivity layer. Therefore, a desirable concentration gradient of the element RCP can be formed easily by using these four elements. This self-diffusion of elements tends to appear more remarkably when the high-resistivity layer is formed at a higher reaction temperature.
In the magnetoresistive device of the present invention, it is preferable that the layer A consists of Fe, Si and Al. Accordingly, a lower resistance of the device can be obtained. Furthermore, the rate of change in the magnetoresistance is increased, and the soft magnetic property of the layer A is enhanced. Thus, a high MR is obtained at a low magnetic field.
In the magnetoresistive device of the present invention, it is preferable that the element RCP forms a compound with the element LONC in the vicinity of the high-resistivity layer in the layer A. Accordingly, the spin polarizability in the vicinity of the high-resistivity layer is increased due to phase separation with the metal element M, so that a magnetoresistive device having a particularly high rate of change in magnetoresistance is obtained.
In the magnetoresistive device of the present invention, it is preferable that the second magnetic layer is formed after forming the high-resistivity layer, and a portion of the high-resistivity layer contacting the second magnetic layer contains an aluminum oxide as a main component. Oxygen and nitrogen ions have a lower diffusion constant in an aluminum oxide than in other oxides. Thus, according to the above structure, diffusion of these ions through the high-resistivity layer can be inhibited effectively when forming the high-resistivity layer. As a result, the probability that the element RCP captures oxygen and nitrogen ions diffusing through the high-resistivity layer increases, so that oxidation and nitriding of the metal element M resulting in spin inversion can be inhibited. Furthermore, according to the above structure, the thickness of the high-resistivity layer is controlled easily. Furthermore, in the above structure, it is preferable that a current is passed so that the first magnetic layer is positive and the second magnetic layer is negative. Accordingly, diffusion of oxygen ions is inhibited when a current is applied, so that the life of the device can be extended.
In the magnetoresistive device of the present invention, it is preferable that at least a portion of the high-resistivity layer is formed by forming a film containing the metal element M and the element RCP, and then reacting the surface of the film with the element LONC. Accordingly, the element RCP in the first magnetic layer and the high-resistivity layer form a thermally stable relationship, so that the reliability of the device at a high temperature is increased. Furthermore, in this case, it is preferable that a current is passed so that the first magnetic layer is negative and the second magnetic layer is positive. Accordingly, diffusion of oxygen ions is inhibited when a current is applied, so that the life of the device can be extended.
Furthermore, the present invention provides a method for producing a magnetoresistive device, including:
(a) forming a first magnetic layer located on a substrate, and a high-resistivity layer located on the first magnetic layer and containing at least one element LONC selected from oxygen, nitrogen and carbon; and
(b) forming a second magnetic layer on the high-resistivity layer;
wherein the first magnetic layer contains at least one metal element M selected from Fe, Ni and Co, and an element RCP different from the metal element M; and the element RCP combines with the element LONC more easily in terms of energy than the metal element M. According to the method of the present invention, a magnetoresistive device of the present invention can be produced.
In the method of the present invention, it is preferable that the step (a) includes:
(a-1) forming a magnetic layer containing the metal element M on the substrate, and
(a-2) forming a layer B containing the element RCP on the magnetic layer, and then reacting a portion of the layer B on the side of its surface with the element LONC thereby to form the high-resistivity layer, and further includes:
after the step (a-2) and before or after the step (b), allowing unreacted parts of the element RCP in the layer B and the metal element M in the magnetic layer to diffuse mutually by heating the substrate at a temperature of at least 50xc2x0 C. but not higher than 350xc2x0 C., thereby to form the first magnetic layer in which the concentration of the element RCP gradually increases to the side of the high-resistivity layer. Accordingly, unreacted parts of the element RCP are allowed to diffuse into the first magnetic layer, so that deletion of spin memory due to unreacted parts of the element RCP can be prevented.
In the method of the present invention, it is preferable that the step (a) includes:
(a-1) forming the first magnetic layer on the substrate; and
(a-2) forming a layer C having a thickness of 0.1 nm to 2 nm and containing the element RCP on the first magnetic layer, and then reacting the layer C with the element LONC thereby to form the high-resistivity layer. Accordingly, when forming the high-resistivity layer, the element RCP in the first magnetic layer captures the element LONC diffusing from the high-resistivity layer to form a compound. Thus, a magnetoresistive device having a low junction resistance and a high MR is obtained.
In the method of the present invention, it is preferable that in the step (a-2), the element RCP in the layer C is allowed to diffuse into the first magnetic layer so that the concentration of the element RCP gradually increases to the side of the high-resistivity layer in the first magnetic layer.
In the method of the present invention, it is preferable that the step (a) includes:
(a-1) forming the first magnetic layer on the substrate;
(a-2) depositing the element RCP on the first magnetic layer in a gas atmosphere containing the element LONC thereby to form the high-resistivity layer. Accordingly, the magnetoresistive device of the present invention can be produced easily.
In the method of the present invention, it is preferable that in the step (a-2), the element RCP is allowed to diffuse into the first magnetic layer so that the concentration of the element RCP gradually increases to the side of the high-resistivity layer in the first magnetic layer.
In the method of the present invention, it is preferable that the step (a) includes:
(a-1) forming a magnetic layer containing the metal element M and the element RCP on the substrate; and
(a-2) reacting the surface of the magnetic layer with the element LONC thereby to form the first magnetic layer and the high-resistivity layer. According to this method, even when the surface of the magnetic layer is not flat, a high-resistivity layer having an approximately uniform thickness can be formed easily. Furthermore, in this method, it is preferable that in the step (a-2), the element RCP is allowed to diffuse into the first magnetic layer so that the concentration of the element RCP gradually increases to the side of the high-resistivity layer in the first magnetic layer. Furthermore, in this case, it is preferable that in the step (a-2), when reacting the surface of the magnetic layer with the element LONC, the surface of the magnetic layer is heated at a temperature of at least 50xc2x0 C. but not higher than 800xc2x0 C. By carrying out such a heating, the diffusion velocity of the element RCP can be increased, and the time for forming the high-resistivity layer can be shortened.
In the method of the present invention, it is preferable that in the step (a), the first magnetic layer is formed by evaporation or sputtering so that the concentration of the element RCP is high on the side of the high-resistivity layer in the first magnetic layer. Accordingly, the concentration distribution of the element RCP in the first magnetic layer can be controlled easily by controlling the deposition rate of the element RCP.
Furthermore, the present invention provides a magnetic component including a magnetoresistive device, wherein the magnetoresistive device is obtained by heating the magnetoresistive device of the present invention at a temperature of at least 200xc2x0 C. According to the magnetic component of the present invention, each magnetic component easily can obtain an arbitrary resistance meeting its use as well as a high MR. For example, a resistance of several ten-ohms to several mega-ohms times square micron for RA (resistance area) is required in MRAM. Also, a resistance of several tens of milli-ohm to several ohms times square micron is required in a magnetic head. Moreover, a high MR can be obtained even when its surface is relatively rough.